In the sector of machines for manufacturing absorbent sanitary articles such as, for example, baby nappies, sanitary towels or the like, wide use is made of ultrasonic sealing systems.
As is known, nappies comprise an absorbent pad or padding which is normally enclosed between a permeable inner layer of non-woven fabric and an impermeable outer layer of polyethylene.
These pads are normally made from a composite web, that is, a multi-layer web, which is cut into lengths.
Absorbent pads of known type comprise, for example, an absorbent core in turn comprising an absorbent material, such as, for example, granules of superabsorbent polymer material (SAP), which may be inside a mixture of containment cellulose pulp (fluff) and absorbent material binder, sandwiched between two layers of non-woven fabric.
At least two layers of the nappy may be joined to each other by seals which are suitably made in such a way as to form, for example, a plurality of rows containing the absorbent material.
These seals can be made by means of ultrasonic systems which, in one embodiment, comprise a contact roller, also known as “anvil”, having on its outer cylindrical surface a pattern of the sealing lines to be made on the finished product.
The layered composite web as described above, from which the nappies or parts thereof are obtained, is made to advance on the surface of the contact roller.
A sonotrode, energized by a source of energy, operates on the web on the side opposite to the anvil in such a way as to make the seals according to the pattern formed on the anvil itself.
In practice, the sonotrode produces friction between the parts of the suitably layered web by means of vibration at ultrasonic frequency opposed by the anvil, in particular at the sealing pattern formed on the anvil. This intense vibration produces heat which causes the materials involved to melt, thereby sealing and assembling the composite web.
One disadvantage of ultrasonic sealing systems of the type described is due to the fact that, in the specific case of absorbent sanitary articles, part of the absorbent material, usually in granular or similar form, inside the composite web may be located along the sealing lines. The seals along these lines are therefore not made properly and are not entirely reliable.